How do you size a distribution transformer for a commercial building?

Transformer sizing starts with calculating total demand load including lighting, receptacles, HVAC, and special loads with appropriate demand factors. Apply NEC optional calculation methods or standard calculation per Article 220. Add 20-25% margin for future growth. Select standard transformer size (75, 112.5, 150, 225, 300 kVA etc.) that exceeds calculated demand. Consider transformer efficiency at actual loading, voltage regulation, and inrush current for sizing upstream protection.